(1) Field of the Invention
The present invention relates to micro electromechanical systems (MEMS) devices and more particularly to a manufacturing process for manufacture of MEMS devices and application of such MEMS devices for a particular use.
(2) Description of Related Art
Micro electromechanical MEMS devices are free-standing structural elements integrated on a substrate. MEMS devices are useful for many sensor or actuator applications such as electrical signal isolators, micro switches, or tuning fork gyroscopes, by way of example. A typical MEMS device has structural elements such as cantilevered beams, suspended platforms, capacitor plates, or other elements displaced from the supporting substrate. The size of these structural elements is typically on the order of millimeters.
The manufacturing process for MEMS devices shares many of the same processing steps employed in the manufacture of integrated circuits, particularly patterning and etching steps. Unlike surface MEMS devices or LIGA devices, a typical bulk MEMS device includes a base substrate that supports the structural element and a sacrificial silicon substrate from which the structural element is obtained. The base substrate may be a Pyrex glass substrate having electrodes and conductive traces deposited thereon. The base substrate may also be etched to include a plurality of pedestals for anchoring the structural elements above the surface of the glass substrate.
The sacrificial silicon substrate has a doped epilayer in which an image of the MEMS device is imprinted using well-known semiconductor lithographic imaging techniques. Portions of the epilayer are then selectively etched using a plasma dry etch, to define the structural elements. The sacrificial silicon substrate and the glass substrate are then aligned and anodically bonded together to form a composite structure with the structural elements of the MEMS device mounted on the pedestals.
Unique to the process for manufacturing bulk MEMS devices, large amounts of sacrificial silicon substrate must then be removed to release the structural elements of the MEMS device. One process for removing the sacrificial portions of the silicon substrate is referred to as a wafer dissolution process. In the dissolution process, the composite structure is immersed in a container of heated solvent to remove the sacrificial silicon substrate. One solvent capable of removing the silicon is a mixture of ethylene diamine and pyrocathecol, commonly referred to as EDP. The doped epi layer has a significantly lower etch rate in EDP compared to the undoped silicon substrate so the silicon substrate is etched at a much faster rate than either the epi or glass substrate. The dissolution method requires that the composite structure remain immersed in the solvent for several hours, depending on etch conditions and substrate size or diameter, to completely remove the sacrificial silicon substrate. Once the sacrificial silicon is removed, the structural elements defined in the epi layer are left suspended above the substrate, but attached to the pedestals.
During the immersion period the solvent is agitated to bathe the composite structure and maintain a high concentration of active solvent in contact with the structure. Unfortunately, the dissolution or dissolving of the substrate in the toxic solvent presents significant environmental and manufacturing problems. For example, since the agitated solvent is heated to about 100xc2x0 C. toxic and corrosive fumes are generated. Thus, containment of the fumes is a necessity for the safety of the manufacturing personnel and provisions must be made to safely vent the fumes from the manufacturing area in a manner that is consistent with environmental and safety concerns. Also, since the composite structure is fairly large, a significant volume of the solvent is required to completely submerge the composite structure. After processing, the spent solvent must be disposed. Clearly, what is needed is a manufacturing method that eliminates the generation of toxic fumes and that minimizes the amount of solvent that is necessary to remove the sacrificial silicon substrate and release the structural elements of the MEMS device.
Another problem with the solvent used in the dissolution method is that endpoint detection requires a visual analysis but visual detection is not possible while the composite structure is immersed because the EDP solvent, in large quantities, is highly opaque. Further, characterizing the etch rate is difficult since the etch rate varies as a function of the concentration of the unspent solvent. Therefore, the time to completely remove the sacrificial substrate will increase as a function of the amount of silicon previously etched. For these reasons, it is necessary for an operator to periodically remove the composite structure from the solvent to visually monitor the etch process. However, this is a noxious process that requires great care on the part of the operator and increases the probability of injury to the operator. Moreover, determining the endpoint of the etch process must be done very quickly before spent solvent coating the partially etched device forms precipitates on the device surface. If the inspection is not performed very rapidly, the precipitates will render the device irreparably damaged and the entire wafer will have to be scrapped. To avoid the formation of precipitates, it is common for the composite structure to be left in the solvent for a longer than optimal period of time before the inspection is performed. Although the risk of precipitate formation is reduced, the extended etch time often results in an over-etched MEMS device that will not function properly. What is needed is a process that permits timely detection of the etch process so that high volumes of composite structures may be completely etched (but not over-etched) regardless of the concentration of the solvent.
After the etch process is complete, the etched composite structure must be cleaned to remove residual solvent adhering to the composite structure. If the solvent is not quickly removed, crystal residue will form as the solvent evaporates. The residual contamination could render the device defective. Accordingly, the dissolution process also includes a cleaning process. The cleaning process requires that the composite structure be immersed in a vat of hot de-ionized (DI) water heated to about 100xc2x0 C. This immersion process subjects the operator to the risk of potential injury from scalding water if the composite structure is not carefully handled.
After the cleaning process, the suspended structural elements are often found to adhere to the glass substrate due to surface tension or stiction (static friction). To overcome the stiction, the dissolution process further includes a vacuum release step where the composite structure is place in a vacuum chamber in an attempt to separate the suspended element from the glass substrate. Often, the vacuum step is not successful, affecting device yields. It has been found that minimizing the amount of the surface area of the glass substrate that could contact the suspended elements, stiction yield loss can be further reduced. For this reason, the prior art dissolution process includes process steps where a plurality of metal stand-offs are formed in the metal under the suspended structural elements. The stand-offs reduce the amount of surface area of the glass substrate that can come in contact with the suspended structural element. Thus, after the DI water clean, the composite structure is immediately placed in the vacuum chamber to rapidly dry and separate the suspended structural element from the electrode since these elements will typically adhere to the glass substrate after the immersion steps. If there is significant delay in removal of the Dl water, the stiction force will permanently maintain the suspended portion in contact with the electrode rendering the MEMS device defective. Although providing the stand-offs require additional processing steps, the improvement in manufacturing yields typically justify such steps. The stand-offs further increase the yield obtained from the vacuum release step, it being noted that neither vacuum release nor stand-offs alone are sufficient to overcome the stiction forces.
The above described dissolution process has poor yield due to poor process control, is very expensive and slow. Further, the process is dangerous in that operators are exposed to toxic fumes and hot liquids. While the above described process is acceptable for research and development or manufacturing small quantities of MEMS devices, scaling the process for large volumes is cost prohibitive. Clearly, what is needed is a process that has improved process control, improved yield, and minimizes the quantities of toxic solvent produced as a by-product of the manufacturing process. Accordingly, what is needed is a process that is controllable, safe and inexpensive for manufacturing high volume of MEMS devices.
The present invention relates to micro electromechanical (MEMS) devices and more specifically to a process for manufacturing MEMS devices. The present invention is a modified dissolution process that removes, in a selective etch step, inactive silicon to release suspended structural elements from a sacrificial silicon substrate using a spray of etchant. Stiction forces are minimized by rapidly switching from the etchant spray to a hot de-ionized (DI) water spray. The use of the two step spray process is critical to the improvements of the present invention.
In accordance with the present invention, the MEMS device includes a Pyrex glass substrate and a sacrificial silicon substrate. The glass substrate is patterned with electrodes, conductive traces and a plurality of pedestals. The sacrificial silicon substrate has a doped epi layer that is selectively etched using a plasma dry etch, to define the suspended structural elements of the MEMS device. The sacrificial silicon substrate and the glass substrate are aligned and anodically bonded into a composite structure with the structural elements of the MEMS device mounted on the pedestals.
A portion of the sacrificial silicon substrate is removed using wet etch of potassium hydroxide (KOH) or backside grind to get to a desired thickness. A combination of both back-side grinding and wet etching may also be used to thin the sacrificial substrate. The remaining portion of the sacrificial silicon substrate is then removed either completely or to an amount sufficient to release the structural element. This removal step uses a commercially available spray acid processing tool. The tool provides a closed chamber in which a one or more composite structures are positioned.
During the dissolution step, an etchant is sprayed onto the composite structure from a plurality of nozzles. The nozzles are positioned to direct the etchant onto the composite structure at a rate sufficient to form a sheeting action on the composite structure. As active etchant is sprayed, spent acid is recovered either for subsequent re-use or for disposal. The progress of the etch process is observable through the windows of the tool since the etchant spray coats the composite structure with a transparent thin sheeting. The etch process is readily terminated since concentration of the etchant is more predictable than the immersion process described above. Advantageously, the present invention does not relay on a mixing action to remove spent solvent from the silicon wafer but rather encompasses a dynamic system where the solvent is constantly circulated. The improved method of the present invention the efficient utilization of the solvent minimizes the amount of solvent required to remove the inactive silicon material from the composite device.
Immediately after the structural elements are released from the sacrificial silicon, a spray of heated DI water is sprayed through a plurality of nozzles onto the composite structure to remove residual etchant. Since the composite structure is cleaned in situ, delays due to transporting the composite structure to the DI water are minimized as is the likelihood that precipitates will form on the composite structure. Also, since the composite structure is not immersed in DI, the occurrence of suspended structural elements adhering to the glass substrate is substantially eliminated. Accordingly, the vacuum drying step of the prior art may be replaced by an air-dry process step.
The illustrated embodiment of the present invention provides improved throughput, improved repeatable and uniform etch rates, a reduction in the number of processing steps and chemical containment for improved safety compared to conventional dissolution processing techniques. Further, since the tool provides an enclosed chamber, escape of noxious fumes is significantly reduced.
Other advantages and aspects of the invention will become apparent to those skilled in the art from the detailed description of the invention which is presented by way of example and not as a limitation of the present invention.